In a vehicle seat hinge assembly, it is common to have several components that comprise the hinge assembly, including the components that provide for the angle of the seatback to be controllably adjustable. Most of these components are mass produced from sheet metal by stamping or similar processes. For any one particular component produced in this manner, a range of tolerances in the order of 0.010" can be expected. Resultingly, replications of the same component will be of slightly different sizes. When the vehicle seat hinge assembly is assembled, the various components are placed seriatim one to another and, resultingly, the aforesaid tolerances of the various parts stack. Thus, the overall dimensions of the assembled vehicle seat hinge assembly may vary by up to about 0.030", or more.
In a vehicle seat hinge assembly having a manually operable release mechanism, the lever arm for releasing the seatback and allowing it to be angularly adjusted and the seatback hinge arm that is rigidly attached to the seatback are generally located at extreme opposite ends of the vehicle seat hinge assembly, with the remaining components located therebetween. The stacked tolerances of all of these components can vary considerably from one replication of a given hinge assembly to the next replication of the same type of hinge assembly, and such stacking of tolerances is most apparent at the extreme opposite ends of the vehicle seat hinge assembly. Further, the tolerances of all of the components in the vehicle seat hinge assembly can stack both negatively and positively to thereby cause an unpredictable angular variation in the orientation of the hand operated lever arm and the seatback hinge arm in either angular direction. This, of course, amplifies the aforesaid angular variation of the lever arm twofold.
Further compounding the problem is the rotatable mounting of the hand operated lever arm, which means that a variation of perhaps up to about 0.025" at the interface between the lever arm and the next serially arranged component that contacts it, typically a latch pawl, can translate to a difference of about 30.degree. to 40.degree. or even more in the angular orientation of the lever arm. An angular variation of this magnitude is unacceptable for aesthetic and functional reasons, and can cause, for example, design problems in the seat frame or seat trim components which must accommodate placement and movement of the lever arm in the finished seat assembly. A range of angular variation of the lever arm of about 2.degree. or 3.degree. is considered acceptable by vehicle manufacturers. The angle of orientation of the lever arm is preferably substantially horizontal, but need not be so.
When the various parts of a seat hinge assembly are assembled, there is an inherent space, known as "clearance" between each part and any part mating therewith. Clearance is due to differences in size between mating parts. Clearances between the mating parts, such as gear teeth, cause a condition known as backlash. Backlash results, for instance when one of the gears in a gear system is rotated and the clearance between any of the mating gears is eliminated such that all of the gears are contacting mating gears so as to allow for concurrent rotational movement of all of the gears. Backlash is a necessary characteristic of all mating gears, and is needed to give the gears enough freedom to mesh and release during the relative motion between any two mating gears.
"Lost motion" is a term used to refer to the amount of travel of a part, typically an actuating means such as a handle or a lever, in a gear system or similar that occurs due to backlash. Basically, there is an amount of motion required in order to temporarily "take up" the clearances between mating parts. This is referred to as lost motion because the motion is not used to perform its intended function. It is preferable in the design of automotive seat hinge assemblies to minimize lost motion in order that the actuating means have a solid feel to the user, and so that only a minimized amount of travel of the actuating parts are necessary to quickly perform their intended latching and unlatching functions. In the present invention, in order to minimize lost motion, the clearances between mating parts can be greatly reduced by moving one of the mating parts at either end of a gear train against the other moving parts, until there is little or no clearance remaining between remaining mating parts in the gear train.
With the vehicular seatback recliner hinge disclosed in applicants' U.S. Pat. No. 5,205,609, issued Apr. 27, 1993, which is incorporated herein by reference, it has been found that, from one mass-produced recliner hinge to the next, constructed as taught thereby, an angular variation in position of the hand operated lever arm due to stacked tolerances as aforesaid in the order of 30.degree.-40.degree. is possible, once the entire recliner hinge has been assembled. Variation of this magnitude is unacceptably high.
In the prior art recliner hinge of U.S. Pat. No. 5,205,609, and in similar vehicle seat hinge assemblies, the locking pawl of the hinge assembly has a cam follower surface that follows a cam surface on the lever means. Where the cam follower surface contacts the cam surface, the cam surface is oriented such that a line perpendicular thereto passes very closely to the axis of rotation of the lever arm, which is typically referred to as a "small moment arm". Such arrangement is desirable for achieving proper engagement of the cam surface of the lever arm with the cam following surface, and for desirable force transmission characteristics between the cam surface and the cam following surface. Such arrangement does, however, further amplify the angular variation in the orientation of the hand operated lever arm, because of the small moment arm involved. A relatively small variation in the linear displacement, perhaps about 0.030", in the cam surface in a direction generally perpendicular thereto will cause a relatively large angular displacement of the lever arm, of perhaps 30.degree. to 40.degree. or even more. In a lever arm having a length of about 2" or 3", the end of the lever arm can resultingly vary in position by as much as 1" or 2". This degree of variation is also unacceptable for the reasons previously given.
Further, because of the large variations in angular displacement of the lever arm attributable to tolerance stacking as aforesaid, the cam follower surface must be designed to accommodate these variations. Thus, the locking pawl is, in such latching mechanisms, predictably unlocked only after about 9.degree. of angular rotation of the cam surface with respect to the cam following surface. While this amount of travel is considered generally acceptable, it would be highly desirable to have the locking pawl unlocked after about 3.degree. of angular rotation, thus increasing the precision and efficiency of the release mechanism. Such increase in precision and efficiency of the release mechanism not only provides for a more precise feel to the user of the mechanism, but allows for closer tolerances to be used in the design of the trim components of the seat assembly which surround and cooperate with the hand-operated lever arm.